A method of this kind is used to a large extent in the automobile accessory industry, where trim strips, window frames and like narrow, and more particularly open, profiled lengths of material which are relatively easy to bend are manufactured.
Such a method is not, however, suitable for stretching and bending closed hollow profiled lengths or half-open profiled lengths of intricate cross-section, because the hollow profiled length is not supported in such a way as to avoid some undesired deformation of the profiled length.
Although this problem can be solved by filling the hollow profiled length with sand where only small batches are to be produced, large batches cannot be produced in this way.
Pipes having relatively thin walls can be bent by means of pipe stretching and bending machines, wherein the pipe to be bent is filled with a mandrel. The mandrel consists of two parts, namely a relatively rigid mandrel shaft which is introduced into the pipe, and a sectional mandrel comprising individual sections pivotably connected to each other and joined together in the form of a chain.
Nevertheless, the two ends of an open-ended pipe, cannot, by the use of such means, be bent symmetrically in the same operation and at the same time. The manner in which the sectional mandrel is pulled through the pipe prevents the sectional mandrel from being inserted from both ends of the pipe simultaneously. The mandrel shaft is kept stationary during the bending process.
Instead of a two-part mandrel, a continuous sectional mandrel may be used, which remains stationary during the bending process. The pipe is moved during the bending operation, whilst the mandrel remains stationary, because the pipe is pulled around a bending tool. The pipe is in practice pulled over the stationary mandrel.
A disadvantage of this method, is that a bending contour cannot be applied to an open-ended pipe from both ends simultaneously and in the same operation, because the pipe is moved around the bending tool.